This invention relates to circular filament lamp, that is to say a lamp in which an incandescent filament such as a coiled wire extends around a generally circular path, within a tubular enclosure. Such a lamp is particularly, although not exclusively, useful in an infrared heater lamp for use in a cooker hob.
Circular halogen infra-red heater elements for cooker hobs are well known, and typically consist of a tungsten filament supported within a quartz envelope. The envelope is in the form of a tube which is bent around to form the major part of a circle, with the ends of the tube being bent outwards and extending generally parallel with a radius of the circle, as depicted in FIG. 1 of the accompanying drawings. The ends of the tube are pinched, during manufacture when the quartz is heated to a plastic state, to form pinch seals. The pinch seals encapsulate the current carrying assemblies for the filament, with each filament end being secured to the assembly inside the quartz envelope, and with a current lead welded to each current carrying assembly outside the envelope. In a cooker hob application, the lamp is contained within a heater pan assembly, which is typically of ceramic material, or which may be in the form of a pressed metal disc, with the ends of the pinch seals and the current leads extending out of the heater pan.
The known lamps suffer from the disadvantages that they are difficult to manufacture due to filament tensions around the small radius bends where the tube ends extend outwards from the circular portion of the lamp, and that the ends of the lamp extend outside the heater pan, with the result that the lamp element does not make maximum use of the area available for the heater pan assembly. Also, there is an unlit portion of the lamp in the vicinity of the tube ends whereby the lamp, in use, provides a broken or interrupted circle of heat (and light). This may give rise to a fall in heating efficiency in the vicinity of the tube ends, as well as being aesthetically unpleasing.
According to an embodiment of the invention, there is provided a circular filament lamp comprising a filament supported within an envelope which comprises a tube with its longitudinal axis formed as the major part of a circle, the ends of the tube terminating in pinch seals through which extend current carrying lead through assemblies connected to the filament, the pinch seals overlapping one another generally circumferentially of the lamp.
The pinch seals may be offset from one another on either side of a plane containing the axis of the filament, or they may be offset in a direction generally radially of said circle.
The upper surfaces of the envelope, in the region of the ends of the tube, may be surface treated to cause them to diffuse radiation from the filament into the unlit regions at the tube ends, thereby giving better distribution of heat and light.
The lamp provides a more compact tube into which it is easier to install the filament; which may be contained completely within its heater pan, thereby optimising the use of space; and which has improved heating efficiency in the region of the ends of the tube.